The present invention relates, in general, to the manufacture of thin-walled containers, and more particularly to methods and apparatus for injection molding generally cylindrical containers having wall thicknesses on the order of 0.015 inch.
As pointed out in U.S. Pat. No. 2,683,899, it is well known in the injection molding art that in the mass production of small tubular articles of plastic, it is difficult to accurately control the wall thickness of the article because the plastic material which is injected into the mold at great pressure has a tendency to build up more rapidly at one side of the mold than at the other. This unequal buildup of plastic material can cause the inner, or core portion of the mold to shift or bend slightly with respect to the outer cavity mold, with the result that articles are produced with irregularities in the wall thickness. If the wall of the article is sufficiently thick, such irregularities may be acceptable for some purposes, but when attempts are made to produce thin-walled articles, such irregularities often result in weakness or incomplete formation of the article. The '899 patent attempted to solve this problem of irregular wall thickness by providing positioning lugs within the mold to hold the core in alignment with the cavity mold. U.S. Pat. No. 3,559,581 recognizes the same problem, and suggests as a solution the use of an annular slit through which a liquid polymer plastic is supplied to the mold cavity. The annular slit is said to cause an even distribution of the plastic around the mold. U.S. Pat. No. 3,829,548 also discusses the problem, and proposes as a solution the provision of a projecting pin which is axially aligned with the mold and which serves to lock the mold parts together to prevent relative shifting.
Although the foregoing patents propose various solutions to the production of injection molded thin-walled containers, they are all directed to articles having shapes which can accommodate the use of alignment pins or lugs, and to articles having wall thicknesses which will tolerate some deflection of the core portion of the mold. However, for very thin walled containers such as plastic cans, or other containers wherein the distance from the injection post to the end of the mold cavity is relatively great, the solutions offered by the foregoing patents are not satisfactory, either because they do not provide sufficient stabilization of the inner core, or because they require an alignment structure which cannot be used with a flat bottomed, closed end container.
The present invention is directed to a solution to the problem of producing a plastic container in an injection mold having a long distance between the injection port and the end of the container cavity, wherein the container has extremely thin walls, on the order of 0.015 inch. Such a container typically has a height greater than its diameter, although other configurations are possible, and may be used, for example, for packaging single-strength juices, frozen concentrates, and like food products, where high internal pressures within the container are not encountered. These containers may be used in conjunction with an external sleeve to enhance the burst strength and to provide a convenient surface for product identification. Because these containers do not require a high wall strength they can be produced with very thin side walls to significantly reduce the amount of plastic used. However, when this is done, it is imperative that the spacing between the inner and outer mold elements be maintained substantially uniform throughout the circumference and length of the container during the molding process, since the thinness of the wall does not provide much tolerance for error. But because of this need for uniformity, such articles are extremely difficult to produce in a high speed injection molding process, since the pressures required for injection of the plastic are extremely high, and some movement of the inner core of the mold with respect to the outer cavity mold is to be expected. Such movement will prevent the plastic from flowing uniformly throughout the mold, resulting in an unacceptably high rate of incompletely molded containers or of containers with extremely thin and weakened wall sections. Thus, the techniques previously used for injection molding of high ratio containers are not suitable for producing containers having wall thicknesses less than about 0.020 inch, and have not been capable of producing wall thickness of 0.015 inch thickness or less.